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a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. featur e ? 4. 75 v t o 16 v su ppl y v o l t a ge o p erat i n g r a nge . ? 38 0 khz ? 1 5 % fi xe d swi t c hi n g f r e que nc y . ? v o ltag e - m od e pw m s t ep -dow n regu lator . ? bu ilt-in po wer mosfet , rds(on) y 10 0m ? . ? 2a out put c u rrent, up t o 91.9% e f ficiency ? 1. 21 5 v ? 1. 5% inter n al refe re nce ? shu t do wn c u rren t less th an 20 ua . ? th erm a l-shu t do wn and b u ilt-in so ft-s tart. ? cycle-b y -cycle cu rren t-lim i t pro t ectio n . ? sho r t circu itry pro t ectio n. ? un de r - v o l t a ge loc k o u t p r ot ec t i on. ? out put v o l t a ge a d just a b l e f r om 1.2 1 5 v t o 16 v . ? sop - 8 pac k a g e. application ? distributed power system s ? battery ch ar g e r . ? palm t op c o m put ers, p d a s . ? dsl m o dem s description a t 17 99 is a vo ltag e - m o d e pw m step- dow n d c -d c co nv er ter cap a b l e of d r iv i n g up to 2a lo ad s ov er a wid e inp u t su pp l y ran g e with ex cellen t line and loa d re gulation characteristics. high ef ficiency is obtaine d through the use of a low rd s(on ) p- ch an n e l po w e r sw i t ch . it uses a fi xed swi t c hi n g f r e que ncy u p t o 38 0 khz t hus al l o wi n g sm all e r si zed c o m pone nt s . t h e out put v o l t a ge l e vel i s use r - p r o gram m a bl e vi a a n ex tern al resistiv e vo ltag e div i d e r . th e bu ilt-in so ft -start can redu ce inru sh cu rren t o n th e in pu t source at t u rn on. the de vi ce al so has bui l t - i n cy cl e- by -cy c l e cu rren t - li m it, th erm a l sh u t do wn , un d e r - vo ltag e l o ck out a n d a n on/ off l o gi c-c ont r o l t h at can po we r d o w n t h e re gul at o r t o a sh ut d o w n m ode . i n sh ut d o w n m ode t h e re gul at o r dra w s l e ss t h a n 2 0 u a o f su pp ly cu rren t . when a sh ort circu it is d e tected on t h e o u t p ut , t h e de vi ce im m e di at el y chan ge swi t c hi n g f r e q uency t o 4 2 k h z an d dut y c y cl e is l i m i t e d t o 5 % t o pr ot ect ic fr om dam a ge. a t 17 99 is av ai lab l e in sop-8 packa g e. block diagram aimtr o n r e s erves the right without notice to change this cir c ui try and specifications. t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 1
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. pin configuration ordering i n formation part number package mar k ing a t 1 7 9 9 s s o p - 8 dat e c o de a t 1 799 s_ g r e s o p - 8 , gr e e n ,date code with one bottom line : d a te c o de * for mor e mark ing information , contact out sales r e pr esentativ e d i r ectly pin descriptions pin n0. sym b ol i / o descripti o n 1 i n p pow e r in pu t. in sup p lies p o w e r to th e i c an d s t ep-d own co nv er ter swi t c h. i n m u st be by passe d wi t h a l o w - es r capaci t o r . 2 e n i en ab le inp u t . en is a d i g ital in pu t th at turn s th e regu lato r on /of f . driv e en h i gh t o turn o n th e reg u l ato r , d r i v e it low to turn it o f f. 3 c o m p o err o r am pl i f i e r out put . a se r i es r c net w or k c o n n ect ed t o t h i s pi n com p ensates a t 1799. 4 f b i feed back i n pu t . c o nnect a re si st i v e di vi der fr om swi t c hi ng o u t p ut t o fb to g r ou nd . 5,6 gn d p gr o u n d . 7,8 s w o po wer s w i t c hi ng o u t p ut . t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 2
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. absolute m aximum ra tings *1 par a m e t e r r a t e d v a l u e unit in , e n voltage -0.3 to 16 v sw vo ltag e -1 to in+1 v fb , c o m p vol t a ge -0 .3 t o 6 v qu iescen t cu rren t 5 ma therm a l resistance (j un ction to am b i en t ja ) * 2 1 0 5
/w ju nct i o n t e m p erat ure (t j ) 15 0
lead t e m p erat ure (s ol de ri n g 10 sec) 26 0
s t ora g e t e m p erature -65~150
hbm 2 kv esd su scep tibilit y* 3 m m 2 0 0 v 1. str e sse s beyond those listed unde r ?absolute m a xi mum ratings? may cause permanen t damage to th e d e vice . th es e are s t res s rati n g s on ly , and functional operation of th e d e vice at these or an y other c o nditions be y o nd those indic a te d in th e op erat i on al s e ction s of th e s p ecif i catio n s is n o t imp l i e d . exp o s u re to absolute max i mum rating co nditions for ext e nded periods may affec t devi ce rel iabilit y . 2 . measured on a pproxim a t e ly 1? s quare of 1 o z . copper fr4 board. 3. dev i ce ar e esd sensitive. handling pr ecaution r ecomm ended. the hu man body mo del is a 100pf cap a citor d i s c h a rged th r o u g h a 1.5k : r e sistor into e a c h pin. recommen d ed operation conditions va l u e s par a m e t e r s y m b o l m i n . t y p . m a x . unit power supp ly v o ltag e in 4. 7 5 1 2 1 6 v ope r ating temperat ure t op -2 0 + 2 5 + 8 5
t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 3
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. electrical c h aracteristics (in=1 2 v , en=5 v , t a =+2 5
, u n less oth e r w is e noted) par a m e t e r s y m b o l t e st c o nditi o n min. t y p . ma x. units i npu t op er ating vo ltag e i n 4. 75 1 2 1 6 v i npu t und er -v oltag e lo ckou t t h res hol d v uvlo in r i si ng 3. 8 4 . 2 4. 6 v uv lo hy stere s is - 5 0 0 - m v qu iescen t curren t iin v fb =1.5v - 3 . 5 - m a sh ut d o w n c u r r e nt i shdn en = g nd - 2 0 5 0 ua oscillato r frequ e n c y f sw t a =+25
- 3 8 0 44 0 khz sh ort - c i rc ui t f r eq ue ncy fb =0 v - 4 2 - khz feedb ack regu latio n vo ltag e v fb t a =+25
1. 20 1. 21 5 1 . 23 3 v fb in pu t bias cu rren t i fb v fb =1.215v -5 0 - 5 0 na cu rren t lim i t 2. 5 2 . 7 5 - a m a x i mu m d u t y c y c l e d main fb=1. 0 v - - 10 0 % m i n i mu m d u ty c y c l e d miin fb=1. 5 v 0 - - % sw switch on-resistan ce r sw - d s ( on ) i sw =2a - 1 0 0 13 0 m sw leaka g e c u rrent i sw in= 1 3. 5v , v sw =gnd , e n =0v - 0 . 1 2 0 p a loa d r e g u l a t i on 0 a t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 4
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. t y pical characteristics e f f i ci ency v s . o u t put c u r r ent and v o l t age 70 75 80 85 90 95 0. 1 0 . 2 0. 3 0 . 5 0. 7 1 1 . 3 1. 5 1 . 7 2 o u t put c u r r ent ( a ) effic i enc y ( % ) 5. 0 v 3. 3 v vi n= 10 v r e ferenc e vol t age v s t e m perat ure 1.215 1.216 1.217 1.218 1.219 1. 2 2 - 2 0 - 1 0 0 1 0 2 5 4 07 08 0 t e m perat ure ( o c) v ref (v ) t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 5
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. load t r a n si ent respo n s e vin=1 2 v , v out=5.0v , l = 2 2uh, iout=50 0 ma to 2.0a ch2:v o ut, ch4:iout, t i me:1ms /div line t r ansie n t re spon se vin=7v to 14 v , v out=5.0v , l=22 uh, iout =20 0 ma ch2:v o ut, ch 3:vin, t i me:0.5s/div t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 6
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. t y pical application cir c uit t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 7
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. general description the a t 1799 features a pw m step-down conve rter operating with a fixed switching frequency of 380 khz and uses intern al power mosfet to provide m a xi m u m ef ficiency . the output voltage of the conve rter can be set as low as 1.215v wit h external resistive divider . w h en trans- conductance signal is higher than saw-tooth wave, then pw m com p arato r will produce on- tim e signal to set internal flip-flop, which turns on the switching power mosfet . the external inductor current ram p s up linearly , sto r ing ener gy in a m a gnetic fi eld. o n ce peak current of flowing through switching power mosfet over current-lim ited threshold or trans-conductance signal level is low e r th an saw- tooth wave, the sw itchin g power m o sfet will turns of f , the flip-flop resets, and external schottky diode turns on. this forces the current through the inductor to ram p back dow n, transferring the ener gy stored in the m a gnetic field t o the output capacitor and load. t o add higher f l exibility to the se lec tio n of externa l com ponent values, the device uses external loop com p ensation. enable contr o l digital logic of en provi d es an electrical on/off control of the power suppl y . connecting this pin to gr ound or to any vol tage less than 1.0v wi ll com p let e ly turn off the regulator . i n this state, current drain fr om the input suppl y is o n l y 20 a, the internal reference, error am plifier , co m p arators, an d biasing circuitr y turn of f . when en control is not r e quired that it should be left open circuited. soft-s tart a t 1799 is built-in soft-st a rt function. when power up, after input voltage is above t h e under - voltage lockout threshold, then sof t -start allows a gradual increase of com p voltage t o get to a stead y state oper a ting poi nt. therefore, reducing i nput sur g e currents. during t h i s ti m e , the co mp voltage increas es and the outpu t duty c y cle is allowed to increase fro m zero to the value required for regulation. the m a xim u m lo ad current is available aft e r the soft-star t cy cle is com p leted. when en pin is taken low , the soft-start capa c i t or is dischar ge to ground . short-cir cuit pr otection if feedback voltage of the regulator falls below 0.7v , the oscillator clock switched operating frequency to 42khz and duty cycle is lim ited to 5% to reduce input power delivered to output. if the short circuitry is not rem oved, the protection circuitry will activa t e sustainability . w h en short- circu it p r obl em is to elim inate, oscillator clo c k will switch back to 380khz autom a tically . t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 8
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. cycle-by-c ycle over -c urr e nt pr otection the a t 1799 provides cycle-by-cycle over - current protection. current lim it is accom p lished using a separate dedicated com p arato r . if the curren t sense am plifier output volta ge is lar g e r than curr en t-lim ited thr e shold lev e l, it will b e immediately turned of f power mos. the cu rrent-lim it feature protects ag ainst a hard short or over current fault at the output. thermal-overload pr otection therm a l-overload protection lim its total power dissipation in the a t 1799 . when the junction tem p eratur e exceeds tj=160 , a thermal sensor act i v ates the ther mal protectio n, which shut s
down the ic, allowing the ic to cool. once the device cools down by 15 , ic will
automatically recover normal operation. for cont inuous operation , do not exceed the absolute maxim u m ju nction-tem p erature rating of tj=150 .
t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 9
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. application s informa t ion external components of step -down converter can be designed by perform i ng si m p le calculation s . it m u st be to f o llow reg u la tion by the output voltage and the m a xi m u m load current, as well as m a xi mum a nd m i ni m u m input voltages. be gin by selecting an inductor value. once l is know , choose the diode and capacitors. inductor inductor selection depends on input vo ltage, output voltage, m a ximum current, switching frequency and availability of i nductor values. the following buck circuit equations are useful in choosing the inductor values based on the application. choose an inductor that does n o t saturate under the m a xi mum r a ting load conditions. t h e m a gnitude of inductance is selected to m a in tain a peak to peak ripple current of 30% of the m a ximum load current. the peak inductor current is given by: in o in lavg l lavg lpeak v v d d i i i i i = = ? + = 2 d is the mo sfet turn on ratio where: i ? l is the inductor peak-to-peak cu rrent ripple and is decided by: osc l o in osc o in l f i d v v l f d l v v i ? ? = ? ? = ? ) ( f osc is the switching frequency . the inductor should be chosen to be able to handle this current and inductor saturation current rating should be greater than i peak . diode selec t ion when the power switch turns of f, the current through the inductor continues to flow . the path for this current is through the diode connected between the switch output and ground. this forward biased diode m u st has a m i nim u m voltage drop and recovery tim e s. schottky diode is recommende d and it should be able to handle those current. as usual, the reverse voltage rating of the diode should be at least 1.3 times greater than the m a xi mum input voltage, and current rating is greater than the t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 10
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. m a xi mum load current. feedback resistor netw ork an external resistor divider is required to divide the output voltage down to the nom i nal reference voltage. current drawn by th e resistor network s hould be lim ited to m a intain the overall con v erter ef ficiency . the maxim u m value of the res i stor n e twork is lim ited by the feedback input bias current and the potential fo r noise being coupled into the feedback pin. a resist or network in the order of 100k ? is recom m e nded. the buck converter output voltage is dete rm ined by the following relationship: ? ? ? ? ? ? ? ? + = 2 3 1 r r v v ref o where v ref =1.215v as specified. input capacitor an input capacito r help s to provide additiona l current to the power supply as well as sm ooth input voltage variations in high current switching regulators. when selecting an input capacitor , a low esr capacitor is requ ired to keep the noise at the ic to a m i ni m u m. ceram i c capacito rs are preferre d, but tantalum or low-esr electrolytic capacitors m a y also suf f i ce. choose an i nput capacitor who? s m a xi mum voltage rating is 1.3 tim es greater than the m a xim u m input voltage, and who? s rms current rating is equal to on e-half of the m a xi m u m dc load cu rrent. it m a y be necess a ry in som e designs to add a sm all valued ceram i c t ype capacitor in parall el with the input capacitor to prevent any ring. output capacitor the output capacitor acts to sm ooth the dc output voltage and also provides ener gy storage. selection of an output capacitor , with an as so ciated equivalent serie s resis t ance (esr), i m pacts both the am ount of output ripple voltage and stability of the control loop . low esr capacitors are preferre d to keep the output voltage ripple low . the output voltage ripple is estim a ted to be: 2 8 ) 1 ( osc out o ripple f c l v d v ? = ? for low esr ceram ic c a pacitors, the output ripple is dom i n ated by the char ging or t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 11
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. dischar g ing of the output capacitor . in the case of low-esr electr oly tic capacitors, output ri pple voltage drop caused by the switching current through the e s r of th e output capacitor . the output ripple is estim ated as : esr l ripple r i v ? ? im pacting frequency stability of the overa ll control loop, the output capacitance, in conjunction with the inducto r , creates a double pole insi de the feedback loop. in addition the capac itanc e and th e es r value c r e a te a ze ro. these f r equ e ncy re spon se ef fects together with the internal freque ncy com p ensation circu itry of a t 1799 m odify the gain and phase shift of the closed loop system . compensation the step-down loop can be com p ensated by adjusting the external com ponent s connected to the comp pin. th e comp pin is c onnected to the ou tput o f the in tern a l trans-conductance error am plifier . the com p ensation capacito r adjusts the low frequency gain , and the series resistor valu e adjusts the high frequency gain. a series capacitor - resisto r combination sets a pole-zero com b ination to control the characteristics of the control sy stem. the system has 2 poles and one zero of im portance, 2 poles consist of com p e n sati on capacitor and output capacitor . one zero is produced by the com p ensation cap acito r and th e com p ensation res i sto r . if electro lytic capacitor with relatively high esr is used, the zero due to the capacitance and esr of the output capacito r can be com p ensated by a third pole set by the compensation resisto r and another compensation capacito r co nnected form comp pin to ground. pcb layout guidelines careful printed circuit layout is extrem el y im portant to avoid causing parasitical capacitance and line inductance. the followi ng layout guidelines are recomm ended t o achieve optimum perfor m a nce. ? please the buck converter diode and induc tor close to the sw pin and no via. ? please ceram i c bypass capacito rs near the input pin. ? locate all feedback resis tive d i viders as cl o s e to their resp ective feedback pins as possible. ? use wide traces and trace length is short as possible. t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 12
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 13
a t 1799 2a s t ep-dow n v o lt age regul ator 2f , no.10, prosperity rd. ii, sci ence-based indust rial park, hsinch u 300,t a iwan, r.o.c. outline 8-pin sop inc h e s m i l l i m e t e r s symbol m i n m a x m i n m a x notes a 0 . 18 8 0. 19 7 4. 8 0 5. 0 0 - b 0 . 14 9 0. 15 8 3. 8 0 4. 0 0 - c 0 . 22 8 0. 24 4 5. 8 0 6. 2 0 - d 0. 05 0 b s c 1. 27 b s c - e 0 . 01 3 0. 02 0 0. 3 3 0. 5 1 - f 0 . 00 4 0. 01 0 0. 1 0 0. 2 5 - h 0 . 05 3 0. 06 9 1. 3 5 1. 7 5 - j 0 . 0 1 1 0. 01 9 0. 2 8 0. 4 8 k 0 . 00 7 0. 01 0 0. 1 9 0. 2 5 m 0 . 01 6 0. 05 0 0. 4 0 1. 2 7 e 1 4 5 o 45 o 1 0 0 8 0 0 0 8 0 - t e l: 886-3 - 56 3-0 878 fax: 886-3 - 5 63-087 9 www : h ttp ://www .ai m tro n . co m . t w 8 / 2 3 / 2 0 0 6 r e v : 1 . 1 e m a i l : s e r v ic e@ a i mtr o n . co m. tw 14

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